The measurement of a subject's visual field or visual field examination is one of the most basic and widely used tests in ophthalmology. The measurement of a subject's visual field is important in the diagnosis and assessment of various diseases or conditions including glaucoma and those neurological diseases which affect the visual system and brain.
A visual field is generally defined as the portion of space in which objects are simultaneously visible to the steadily fixating eye. Abstractly, the visual field can be thought of as somewhat more than one half of a hollow sphere situated before and around each eye of a subject. Within this portion of the sphere, objects are perceived while the eye is fixating, on a stationary point on the inner surface of the sphere. Objects that are visible on the inner surface of this portion of the sphere stimulate various portions of the retina and are then conducted through nerve fiber bundles and eventually stimulate the visual cortex of the brain. Visual field testing can reveal the amount of damage created by trauma or disease along the visual pathway from the retina to the visual center in the brain.
Traditional methods of testing and measuring visual field have required a great deal of concentration from the subject being tested. The test subject was required to subjectively respond, either visually, physically, or by means of an indicating device, that he has or has not sighted a target at a particular location in his visual field.
The delay between stimulation of the retina and the response of the person being tested results in a loss of spontaneity and creates both false positive and false negative results. The traditional methods of testing and measuring visual field require the subject to fixate on a central point while an examiner either moves objects through the subject's field of vision or various lights or stimuli would be presented within the subject's field of vision to which the subject would be asked to respond to. From these methods, plots of the location of each point seen by the subject are obtained, from which, the subject's field of vision can be mapped.
The primary difficulty associated with these traditional methods of measuring a subject's visual field is that the subject is required to fixate or gaze toward a central fixation point while the stimuli are presented within the subject's visual field. This requirement that the subject must fixate or gaze on a central fixation point leads to a great deal of fatigue, inaccuracy, and lack of repeatability due to false-positive or false-negative responses due to the natural tendency or reflex of the human eye to move towards a new stimuli within its visual field. When a stimulus enters the visual field of a subject, the subject has a tendency to move their eyes toward the new stimulus and away from the central fixation point or reference point.
In order to overcome the uncertainties and difficulties in the foregoing testing techniques, attempts have been made to utilize the natural characteristic voluntary or involuntary eye movements which results when a target is presented to a subject within his visual field. Such methods and apparatuses are disclosed in U.S. Pat. No. 3,827,789 to Molner et al., issued Aug. 6, 1974, and U.S. Pat. No. 4,059,348 to Jernigan, issued Nov. 22. 1977. The Molner et al. patent describes a method and apparatus for measuring a subject's visual field by presenting a target consisting of a spot of light at a series of selected locations within the subject's visual field and monitoring and interpreting the subject's resulting eye movements and positions. However, the Molner et al. patent teaches that in order to ensure accurate test results, it is essential that the subject be looking directly at a known reference or central fixation point on a screen at the time that the target is presented to the subject on the screen. When a spot or dot of light suddenly appears, the subject's eyes respond to the sudden appearance of the dot of light by directing the gaze and fixating on that target.
The Molner et al. patent teaches an apparatus which is employed to measure the subject's visual field and to make a permanent photographic image thereof including the targets which the subject sees as determined by an eye movement monitor and its associated logic on circuitry. However, the method disclosed in the Molner et al. patent requires the subject to fixate on a central fixation point which, as described above, has certain disadvantageous properties associated therewith. Furthermore, the method disclosed in Molner et al. merely detects eye movement and not actually tracking the position and location of the eye in response to a stimulus. By relying only on eye movement as an indicator of those points or stimuli, the method disclosed in Molner et al. only allows for a yes or no answer regarding the subject's departure from the fixation point to a newly presented target stimulus without giving an actual anatomical point on the retina which has been stimulated thereby facilitating exact mapping of the retina and subject's visual field.
Therefore it would be advantageous and desirable to have a method and apparatus for evaluating the visual field of a subject which eliminates the disadvantages set forth above and which provides more accurate and versatile test data which may be used for other applications.